The present invention relates in general to multi-stage optical amplifiers. More particularly, the present invention relates to a multi-stage erbium-doped fiber optical amplifiers having high output power and low noise.
One technique used to increase the capacity of a fiber optic communication systems is wavelength division multiplexing (WDM). In a WDM system, the total signal output power of an optical amplifier generally increases with the number of total channel counts. Use of an extended wavelength band sometimes referred to as the L-band (approximately 1565 nm to approximately 1620 nm) is also being implemented to further increase channel total counts. As a result, there is a renewed need for higher power, L-band optical fiber amplifiers, such as high-power erbium-doped fiber amplifiers (EDFAs) with a good performance. EDFAs have to a large extent replaced the current optoelectronics regenerators in many optical long haul terrestrial and undersea lightwave communications systems. EDFAs may be used as power amplifiers to boost transmitter power; as preamplifiers to increase the signal amplitude received by the receiver; and as in-line repeaters to periodically boost the signal to a level sufficient to traverse long span distances.
Optical fiber amplifiers are designed by considering a number of parameters including gain, output power, and noise performance. Noise performance is typically measured by the noise figure (NF) which is defined as the signal-to-noise ratio at the input of the optical amplifier divided by that at the output. The noise figure NF is measured in dB. When optical fiber amplifiers are used as repeaters, it is desirable that these operate with a very low noise figure and a high output power in order to maximize the distance between adjacent repeaters in the lightwave system. High output is also required where repeaters are used in systems employing multiple multiplexed channels.
Optical fiber amplifiers often have multiple gain or amplification stages; for example an input stage and a power stage. Typically, pump lasers providing light in wavelength bands centered about 980 nm and 1480 nm are used to pump both the input stage and the power stage. However, the power stage typically requires significant pump power to amplify the signals, and it is technically challenging and costly to implement single-transverse-mode laser sources at sufficient power at these low wavelengths. As a result, the power stage of a high power optical fiber amplifier is often very complex in order to accommodate multiple pump sources and to provide the desired high signal output powers. Accordingly, a need exists for an optical fiber amplifier that has a less complex and less expensive power stage.
The present invention is directed to a multi-stage optical fiber amplifier that includes: a first gain stage having an input port and amplifying an optical signal; a pump source coupled to the first gain stage; and a second gain stage coupled to the first gain stage. the second gain stage further amplifies the optical signal to a desired output signal power. A second pumping source is coupled to the second gain stage. The second pump source is an optical medium doped with a rare earth element.
According to one embodiment of the present invention, the second pumping source is a fiber laser that comprises a rare earth element, such as erbium or thulium, and has an operating wavelength between about 1520 nm and about 1620 nm. Preferably, the operating wavelength is about 1535 nm.
According to an embodiment of the present invention, the second pumping source comprises at least one light source having a wavelength between about 1500 nm and about a wavelength that is 1 nm shorter than the shortest wavelength of the output signal.
According to one embodiment of the present invention, a multi-stage erbium optical amplifier comprises a first stage that is pumped by a first light source capable of producing an inversion of at least 0.6 at one of the ends of the first stage, and a second stage that is pumped by a second light source in which the inversion is substantially reduced at the second stage so that gain at the extended band (or L-band of an EDFA) is produced.
It is an advantage of the multi-stage optical fiber amplifier of the present invention that it utilizes a relatively inexpensive and simple power stage. It is another advantage of the present invention that less power is required to operate the pump coupled to the power stage of the optical fiber amplifier. The foregoing and other aspects of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.